Most of the infected computers we see are infected with Adware
Viruses.  These programs are usually downloaded with some kind of
special video player, music player and the like.  They have varying
effects, but typically bring you pop up ads saying your computer is
infected or steal your browser home page and direct you to one of
their sites.  Adware viruses have gotten increasingly more
sophisticated.  They use pop up windows that imply that the warning is
part of Windows, which they never are.  A common term used is
"Windows Security Center" to get you to download their scanner.  The
virus will then scan your computer with bogus results and try to get you
to register the program for $50.00 so it will remove them.  I call them
blackmail programs and they’re very common.  If you’re receiving pop
ups such as these and you don’t have Internet Explorer (or another
browser) open than you are already infected and these pop ups are
being generated by your computer.  In addition, if you have flashing
symbols, usually at the right hand bottom of your computer screen
telling you that you’re infected it is also being generated by your
computer.  These viruses eventually slow your computer down and will
sometimes make it so that you cannot get onto the Internet.  In the
worse cases, Windows won’t boot up.  Don’t fear that they will
damage your computer however, they can always be removed and
your data is usually safe.  
Virus, Adware & Spyware